Many different techniques for producing optical devices on a surface are known. For example, there are various well known techniques for producing metrological scales, for example phase scales or amplitude scales.
Amplitude scales usually comprise a surface that includes features that determine the amplitude of light that, in operation, is received by a readhead from the scale. For example, a reflective amplitude scale can include a sequence of reflective and non-reflective lines formed at accurately determined positions on the surface. As the readhead is moved over the surface it can determine its position accurately based on the location and number of the reflective and non-reflective lines. In general, although the transverse position of the reflective and non-reflective lines on the surface must be determined accurately, the vertical profile of the reflective and non-reflective lines above the surface is of secondary significance for an amplitude scale device. A variety of techniques can be used to produce amplitude scale devices.
Phase scales have a sequence of marks distributed over a surface, with each mark having an accurately determined height, and being formed to provide wells of accurately determined depth between each mark. In operation, light applied by a readhead reflects both from the top of each mark and from the wells between marks, and the readhead is able to detect either constructive or destructive interference of reflected light, using known techniques. Typically, the marks are formed to provide a rectangular wave shaped structure on the surface, with the depth of the wells between marks usually being equal to half a wavelength of the light applied by the readhead. Variations in the height of the marks, or the depths of the wells, or the presence of significant roughness on the surface can cause a significant worsening in signal to noise ratios that are obtainable by readhead measurements on the phase scale. Therefore, phase scales are typically formed using photolithography techniques which, whilst they can be time consuming, are able to provide surface features of sufficient resolution.
It has been suggested to use laser ablation techniques to remove significant quantities of material from a surface to form phase scale features. However such laser ablation techniques can result in the wells having rough surfaces, affecting their reflectivity and increasing signal to noise ratio. Furthermore, if significant quantities of material are ablated from the surface it has been found that some of the material can settle elsewhere on the surface and thus interfere with operation of the phase scale.
An alternative method of forming a phase scale device has been described in WO 2006/120440 in the name of the present applicant. According to that method, a laser beam of suitable intensity is applied to a scale substrate and causes the softening and displacement of substrate material away from the focal point of the laser beam, without substantial removal of material. The action of the laser beam does not degrade the reflectivity, with each point on the scale remaining reflective and without substantial roughening. The beam is applied repeatedly at different locations across the substrate to build up a phase scale device having a desired profile. The resulting profile has marks and wells with rounded edges, but it was found that despite having such a rounded, rather than square wave, profile the resulting device could still function satisfactorily as a phase scale device. The process described in WO 2006/120440 can be more rapid and efficient, and cheaper to implement, than photolithographic techniques.
The application of a laser beam to a surface to move material to form an optical device having a desired profile is described in S F Rastopov and A T Sukhodol'ski{hacek over (i)}, 1987 Sov. J. Quantum Electron. Vol 17, 1091. The method described in that document was used to form a diffraction grating. A thin liquid layer of a binary liquid solution was provided on a surface, and laser radiation was applied, which caused local evaporation of volatiles from the solution driven by local heating, which in turn caused mass transfer that caused the formation of a surface profile of material. The laser radiation was applied repeatedly to build up a desired profile forming a diffraction grating structure. The mass transfer was attributed to the Marangoni effect (also variously called the Gibbs-Marangoni effect, Marangoni convection and thermocapillary convection), which is a physical phenomenon whereby a surface tension gradient at the interface between two fluids causes mass transfer.
U.S. Pat. No. 5,907,144 describes the use of thermocapillary or surface tension forces caused by application of a pulsed laser beam to a metal surface on the edge of a magnetic disk, to cause the flow of metal away from the irradiated area. The laser beam is applied at different locations to form a curved reflector that comprises a bar code structure.
The welding literature describes methods for manipulating the profile of work pieces, including causing the sculpting of material by suitable application of laser radiation under controlled conditions. It is known from welding literature that surface tension gradients of liquid metal can be altered or reversed by varying the oxygen or sulphur content of the metal. It is also known to use a CO2 assist gas during laser processing, which can alter the oxygen content of the metal. However, the welding literature is generally concerned with the formation of large scale mechanical bonds or structures rather than the formation of optical devices. In many cases, surface tension effects can be undesirable in the context of welding.
It is an aim of the present invention to provide an improved or at least alternative method of forming an optical device.